1. Technical Field
The present invention generally relates to wireless communication systems and in particular to signaling in wireless communication systems.
2. Description of the Related Art
The air interface using Orthogonal Frequency Division Multiple Access (OFDMA) physical layer (PHY) in 802.16e Worldwide Interoperability for Microwave Access (WiMAX) system is typically split into 5 ms frames comprising of a downlink (DL) sub-frame and an uplink (UL) sub-frame. The DL sub-frame is further divided into regions. The Institute of Electrical and Electronics Engineers (IEEE) 802.16e specification defines the regions to be independent rectangular collection of slots (i.e., a minimal unit of time-frequency resource used to carry data in OFDMA PHY). Each Hybrid Automatic Repeat Request (HARQ) region definition and sub-burst allocation within the HARQ region adds DL MAP overhead, thus expanding the DL MAP (i.e., a collection of slots carrying control information to allocate DL/UL bandwidth) which occupies DL slots in a vertical raster fashion as depicted in FIG. 2. The HARQ region definition includes a starting symbol and starting sub-channel number for the HARQ region, a number of symbols and a number of sub-channels to define the dimensions of the region, and a duration, in slots, for each sub-burst in the sequence the sub-bursts are allocated within the HARQ region. One HARQ region accommodates a maximum of 15 (16 with 802.16e Corrigendum 2) HARQ sub-bursts.
The majority of the mobile station/user (MS) population in WiMAX is expected to be either HARQ capable (i.e., able to perform HARQ retransmissions and combining) or capable of decoding the HARQ region. Therefore most of the allocations (except the broadcast data and DL data during the initial network entry procedure when new MS capability is unknown) are expected to be in the DL HARQ regions.
The rectangular shape of the HARQ regions and the limit of 15 (16 with 802.16e Corrigendum 2) sub-bursts per region lead to slot wastage in individual HARQ regions. This slot wastage increases as more HARQ regions are defined. The limit of concurrent bursts (number of bursts that may be assigned to an MS within a symbol time per DL frame) aggravates this problem as more HARQ regions need to be defined even when the number of sub-bursts per region limit is not met for an existing HARQ region.
Typically “stacking” (i.e., a technique by which individual HARQ regions are placed on top of each other in sub-channel dimension) may be used to minimize this slot wastage. However a transmit Adaptive Array (TxAA) system requires each HARQ sub-burst to range the entire range of major sub-channel groups, a predefined collection of 4 or 6 sub-channels. Consequently, stacking of HARQ regions within the same slot duration is very complex. The HARQ regions typically span the entire range of sub-channels available within the DL sub-frame. When TxAA diversity is enabled this slot wastage is even more significant.